Meindert Sprang wrote:
These figures are not realistic. Up to the point of the hull speed they
resemble reality, but as soon as the hull starts planing, more 'ordinary'
rules of resistance/drag apply and as far as I know the function becomes
more like a square root function, which is mathematically definately not an
asymptote.

Well, there's your mistake. A hull under planing conditions is subject
to normal drag, including air resistance. It no longer is subject to
wave-making resistance, but that doesn't mean that *all* resistance goes
away. And the function for increase is a multiple of velocity squared,
which will have an asymptote.

DSK